Dengue haemorrhagic fever (DHF), the most severe form of illness following infection with a dengue virus, is characterized by plasma leakage and a period of increased microvascular permeability. Monitoring of plasma volume and body fluid compartment shifts is an integral part of the clinical management of DHF, and is crucial to the performance of clinical research studies on DHF pathogenesis. Multifrequency bioelectrical impedance spectroscopy (BIS) was assessed as a non-invasive method to monitor body fluid compartment shifts in children participating in a prospective, hospital-based, study of dengue virus infections in Thailand. Over the 48 h surrounding defervescence, the extracellular water/intracellular water ratio (ECW/ICW) rose in children with dengue virus infections and correlated with increasing disease severity [DHF > intermediate dengue fever (DF)/DHF > DF]. Plasma leakage remained within the ECW compartment and was not directly measured by multifrequency BIS. Expansion of the ECW space in DHF appeared to be primarily due to diminished renal water clearance. During the course of dengue illness, multifrequency BIS did not improve on serial haematocrit and bodyweight determinations for monitoring plasma volume contraction and ECW expansion, respectively.